The present invention relates to an electrical bypass device for bypassing and isolating a defective module of a battery.
Typically, a battery comprises a plurality of series-connected modules, each module comprising a plurality of series and/or parallel-connected electrochemical secondary cells. A battery is generally designed to operate under so-called nominal conditions, in other words inside of given power, voltage and current ranges. When one of the modules of the battery becomes defective as a result, for example, of ageing of certain secondary cells or through use outside of nominal conditions, internal resistance increases. When a defective module is in series with other modules that are operational, the high internal resistance of the defective module leads to the whole battery becoming non-operational, even if the number of non-defective modules is sufficient to keep the battery working in a slightly degraded operating mode. For very costly high power batteries for which replacement is difficult, isolating the defective module is a necessity. The use of actuators is known for isolating and bypassing a defective module to allow the battery to continue operating. As a defective module can in general not be repaired, such actuators are generally one-way single-use actuators.
FIGS. 1a and 1b are schematic diagrams of a frangible actuator as disclosed in French patent FR-A-2,776,434 (equivalent to U.S. Pat. No. 6,249,063 B1) at respectively the non-actuated and actuated position. The diagrams of FIGS. 1a and 1b are intentionally simplified to facilitate understanding of the principle of switching of the switches. Actuator 10 comprises a first, second and third power terminal respectively bearing reference numerals 1, 2, 3. Actuator 10 also comprises a plunger 4 including a switching portion 14. Plunger 4 is movable between two extreme positions, a first position in which power terminals 2 and 3 are electrically connected by switching portion 14 which we shall refer to below as the “connection position”, and a second position in which it the power terminals 1 and 3 which are electrically connected by switching portion 14, which we call below the “isolating position”. Actuator 10 is shown in the connection position in FIG. 1a and in the isolating position on FIG. 1b. Actuator 10 also comprises a frangible retaining member 5 which retains plunger 4 in the connection position. Retaining member 5 is kept closed by a fusible wire which melts when the battery cell module fails.
Actuator 10 also comprises a spring 6 which is compressed in the connection position and which urges plunger 4 to the isolating position. When the fusible wire melts, retaining member 5 get separated and no longer restrains plunger 4, plunger 4 is then slid to the isolating position through the action of spring 6.
In the connection position, changeoverswitch 14 makes switch 2-3 between the second actuator terminal 2 and the third actuator terminal 3. In the isolating position, changeover switch 14 makes switch 1-3 between first actuator terminal 1 and third actuator terminal 3.
When the actuator is connected to a module, the connection position corresponds to connection of the module in series with other modules, and the isolating position corresponds to an isolation of one terminal of a module, and to the module being bypassed. The actuator is connected to a module by electrically connecting first actuator terminal 1 and second actuator terminal 2 to the terminals of the secondary cells and connecting third actuator terminal 3 to a terminal of the following or preceding module.
FIG. 2a and FIG. 2b are circuit diagrams showing a module 7 connected to an actuator as described above. The first actuator terminal 1 is connected to a first terminal (positive terminal in diagram 2a, negative terminal in diagram 2b) of module 7 but also to an opposite polarity terminal (a negative terminal in diagram 2a, positive in diagram 2b) of a following (diagram 2a) or preceding (diagram 2b) module, connected in series with module 7. The second terminal 2 is connected to the other terminal (the negative terminal in diagram 2a, the positive terminal in diagram 2b) of module 7. The third terminal 3 is connected to a terminal of opposite polarity to that of the terminal connected to second terminal 2 of a preceding or following module. The preceding or following module connected to third terminal 3 is series connected to module 7 by the said switch 2-3. If module 7 is a first or last module of the battery, third actuator terminal 3 or first actuator terminal 1 is connected to one of the battery terminals.
The polarity of the terminals of module 7 connected to the first and second actuator terminal can also be reversed. FIG. 2a is an electrical circuit diagram in which the switch 2-3 is in series between the negative terminal of module 7 and the positive terminal of the preceding module 8 whereas in FIG. 2b an electrical circuit diagram is shown in which the switch 2-3 is in series between the positive terminal of module 7 and the negative terminal of the next module 9.
Thus, in electrical circuit diagram 2a, when the plunger is in the connection position, the normally closed switch 2-3 is in series between module 7 and a module 8 that precedes it. Similarly, the normally open switch 1-3 is in parallel with the series connection of module 7 and normally closed switch 2-3.
This means that when plunger 4 is in the connection position, module 7 is in series between the preceding module 8 and the module 9 that follows it via the switch 2-3 of the actuator. When module 7 fails, retaining member 5 separates and plunger 4 moves from the connection position to the isolating position under the influence of spring 6. In this way, the switch 2-3 gets broken off and isolates the terminal (the negative terminal in diagram 2a, the positive one in diagram 2b) of module 7 connected to second actuator terminal 2. The change of position of plunger 4 and switch 14 also closes the switch 1-3. Module 7 is now isolated and the modules that precede and follow it are connected in series by the switch 1-3 of the actuator.
The actuator as described above thus makes it possible to isolate and bypass a module that has failed in a battery, setting up an electrical circuit which bypasses and isolates this module.
There is an increasing need for batteries that supply higher power, for example for applications in the satellite field. To provide batteries supplying heavy currents, the number of secondary cells in parallel in each module is increased.
As illustrated in FIGS. 2a and 2b, each positive and negative terminal of the secondary cells is connected either to the first actuator terminal 1 or to the second actuator terminal 2 by stranded cable. Now, as is known, the heavier the current passing through the strands of a cable, the greater the amount of heat generated. Standards, such as European stand ECSS (European Co-operation on Space Standardization) Q30 11 A concerning derating of electrical, electronic and electromechanical components used for applications in the satellite domain, impose a minimum cross-section on stranded cable for a maximum current passing therethrough. Such standards are becoming even stricter, meaning that stranded wire cables need to have an even greater cross-section for a given current.
When the cross-section of stranded cables is increased, this has the effect of increasing battery weight, the latter being a determining factor for applications in the satellite domain. Further, when stranded cable cross-section is increased, this leads to increased stiffness thereby accentuating difficulties in cabling, and is detrimental to battery compactness.
One solution consists in using two cable runs in parallel, in other words connecting each secondary battery terminal using two separate stranded cables. In this case, the module would include two pairs of terminals each pair consisting of a positive and a negative terminal. Using two cable runs in this way makes it necessary to install two actuators for isolating and bypassing a module which has failed.
Now, if one of these actuators were to operate inadvertently or erroneously, without the other actuator is triggered, we would be faced with a short circuit at the module terminals. Indeed, in such a case, current could flow between one (for example positive) terminal connected to the normally closed switch of the actuator which has not triggered, and the other (for example negative) terminal connected to the closed switch of the triggered actuator, thereby setting up a short circuit between the positive and negative terminals of the module by passing via one terminal of the preceding module to the next one. Such short circuit could be a source of fire.